1. Field of the Invention
The present invention is directed to electronic circuitry for controlling the speed of a subfractional horsepower DC motor having a switch which repeatedly interrupts the motor current to generate a pulse-like current, with the motor EMF being monitored synchronously with the interruption of the motor current, i.e., after each interruption, with the sensed value being supplied to a comparator, with the output of the comparator being used to control a driver stage which includes the switch.
2. Description of the Prior Art
Subfractional horsepower DC motors ar used for driving small tools such as dental instruments, such as drills, grinders and the like. These motors operate in a voltage range of up to 24 volts. To permit such tools to operate within a large speed range (between a few hundred and 160,000 rpm), the motor must be similarly adjustable in speed over a wide range. In a known subfractional horsepower DC motor of this type, it is possible to vary the speed between about 2,000 and 40,000 rpm. Different gearing reductions and increases can be used to further decrease the minimum speed, or further increase the maximum speed, at the point of attachment of a tool. Such gearing can be attached to the drive shaft of the motor or at the head of a handpiece which accepts the tools.
German OS No. 32 21 146 discloses a circuit of the type described above wherein the speed is controlled by clocking the operating voltage and undertaking an EMF measurement during the pauses between the pulses. An interrupt circuit is provided in the motor drive circuit for this purpose. A sample and hold circuit samples and stores the motor voltage, and a regulator is provided which is supplied with a rated value and the actual value of the motor voltage from the sample and hold circuit. A timing circuit is also provided which activates the interrupt circuit to repeatedly briefly interrupt the motor current, and which also activates the sample and hold circuit to cause the value of the motor voltage to be sampled with a slight delay following the beginning of the interruption of the motor current, but before resumption of the motor current, i.e., before resumption of the leading edge of the next current pulse.
In this known circuit, the motor is operated as a generator in the interrupt phase, and the EMF of the motor-acting-as-generator is used as the actual value for the control circuit. Interruption of the motor power supply is timed to begin after the field dismantling phase. In this known circuit, the sampling frequency or repetition rate is between 10 and 1,000 Hz, and the sampling time is between 5 and 30 microseconds given a turn-off time of the motor current of from 250 through 500 microseconds. This known circuit is comparatively complex, particularly because a controllable voltage source must be provided, with a relatively expensive power stage, which must be designed to accommodate substantially the entire operating power.